The resolution of a solid-state imager depends upon how many discrete solid-state photosensors are included in its imaging area. Some solid-state imagers do not include enough discrete photosensors for obtaining a high resolution television signal suitable for commercial broadcast applications. Although imagers having a greater number of discrete photosensors can be manufactured, the processing yield is relatively low due to imager (photosensor) defects which greatly increase in number with modest increases in the number of discrete photosensors in a given imaging area. Therefore, it is desirable to have an imaging arrangement which provides high resolution image signals while utilizing solid-state imagers having less than the number of discrete photosensors conventionally required.
To increase the apparent resolution of a solid-state imager, it is known to split light representative of an image into two light images and dispose two solid-state imagers so as to receive respective ones of the images. The imagers are positioned so that the image received by one of the imagers is spatially offset or displaced one-half of the width of an individual image photosensor (pixel) with respect to the image received by the other imager. This spatial offset results in the sample signals supplied by the imagers to have carrier and sideband components 180.degree. out-of-phase with respect to each other. A time delay at the output of one of the imagers is used to temporally align the imager output sample signals in a manner so as to compensate for the spatial offset. The imager output signals are then combined whereby the mutually out-of-phase carrier and alias components cancel each other and result in a signal having an apparent increase in its resolution. This technique is applied to a color television camera including three solid-state imagers in U.S. Pat. No. 4,334,238 issued June 8, 1982, to Morishita et al., and assigned to Nippon Electric Company. A block diagram of this camera is shown in FIG. 1 herein and includes a lens 10 for focusing an image of an object 12 through a beamsplitting prism 14 onto the photosensitive imaging area of three solid-state imagers 16, 18 and 20. Beamsplitting prism 14 includes prism blocks 22, 24 and 26 for coupling the green (G) light from the object through a boundary surface 28 while the red (R) and blue (B) light are reflected from surface 28. The green light passing through surface 28 is equally divided by a partially reflecting/transmitting surface 30, such as a half-silvered mirror, with one-half the green light passing through surface 30 to imager 18 and the other half being reflected by surfaces 30 and 28 to imager 16. Imagers 16 and 18 are physically positioned with respect to their received focused images so that a point in the image focused on imager 18 is spatially displaced one-half of a pixel width with respect to the positioning of the corresponding point in the image focused in imager 16. Due to this one-half pixel offset, the carrier and sideband portions of the image-representative sample signals supplied by imagers 16 and 18 are 180.degree. out-of-phase with respect to each other while the baseband signal portions remain unchanged. These green signals are temporally aligned by sampling them with circuits 32 and 34, respectively, which receive 180.degree. out-of-phase sampling signals derived by an oscillator 36 in conjunction with a sampling pulse generator 38. The output of sampling circuits 32 and 34 are combined thereby cancelling the mutually out-of-phase alias components and resulting in an apparent improvement in resolution, and applied to a green signal processor 40 for developing the green video signal.
The red and blue light reflected from surface 28 exits from prism block 26 and passes through an optical low-pass filter 42 and a red/blue color encoding filter 44 before reaching imager 20. Optical filter 42 reduces the amount of spurious signals produced in the output of imager 20 due to aliasing between the optical image and the imager pixels. Color encoding filter 44 allows the red component of the light to fall on alternate pixels of imager 20 while blocking the blue light and allows the blue light to fall on the remaining pixels while blocking the red light. Thus, imager 20 provides an output signal which alternately (on a pixel-by-pixel basis) represents the red and blue color components, respectively, of the light received from object 12. Sampling circuits 46 and 48 sample the red and blue components, respectively, of the output signal from imager 20 in response to 180.degree. out-of-phase sampling signals provided from sampling pulse generator 50 for developing, in conjunction with red and blue processors 52 and 54, respectively, red and blue video signals. Although this arrangement improves the apparent resolution of the green signal component, which is desirable since image pick-up devices are typically more sensitive to this color component than to the red or blue color components, the use of a color encoding filter is disadvantageous because it reduces the light sensitivity for the red and blue color components by 50% due to transmission losses. Furthermore, this arrangement requires precision registration of the color encoding filter with the individual pixels of the third imager. Additionally, since the red and blue signals are derived from alternate pixels of a single imager, these signals necessarily have a reduced resolution with respect to the signals which would be provided if they were each derived from their own imager.
U.S. Pat. No. 4,507,679 issued to Bendell on Mar. 26, 1985, and assigned, like the present application, to RCA Corporation, relates to a color television camera including a beamsplitter having four output ports which improves the apparent resolution of one of the color signals without reducing the resolution of the others. Specifically, two output ports are used for deriving a high resolution green signal in a manner substantially the same as previously described and red and blue signals are each derived from their own solid-state imagers associated with corresponding third and fourth output ports of the beamsplitter. Although this arrangement provides an improved resolution green video signal and red and blue video signals without reduced resolution, a four port prism and four solid-state imagers are required.
Thus, there is a need for a camera which improves the apparent resolution of one of the color signals without reducing the resolution of the others but which requires a beamsplitter arrangement having only three output ports and only three solid-state imagers. Such a three-port arrangement is less complicated than a four-port arrangement and also consumes less power.